It is well known to feed burners, for turbine generator boilers with fuel in the form of airborne particulate coal; structures for carrying out this function are commonly found in electric utility plants throughout the United States and Canada. It is typical in these structures to use a main supply conduit to receive finely pulverized coal from a pulverizer/classifier. It is also typical to divide the main supply conduit into several parallel branches which are connected to individual nozzles or injectors arranged at spaced points around the burner so as to project a stream of particulate coal toward a fireball which is essentially centralized within the burner.
A problem which arises in systems of the type described above is insuring that the branch conduits provide at least approximately equal coal flow rates so that the fireball is stabilized as to size and location and receives essentially equal inputs of fuel from all of the nozzles. The branch conduits are of different lengths and the longer ones present greater resistance to flow. Therefore a variety of devices, hereinafter referred to as “variable orifice” devices, have been created to introduce some additional flow resistance into shorter branches thereby to balance the flows of particulate coal as between parallel branch conduits feeding a single burner. Examples of variable orifices are found in U.S. Pat. Nos. 5,873,156, 6,055,914, 6,186,079, 6,257,415 and 6,234,090.